.gitignore: Reorganize

Divide .gitignore into logical sections, and be explicit about files
that only need to be ignored at top level.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
.gitignore: Add compile

This is pulled in by Automake ≥ 1.14.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
.gitignore: Add other kinds of tags

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
perl/modules/.gitignore: Add META.json, MYMETA.json

ExtUtils-MakeMaker 6.57_07 and later generate META.json and
MYMETA.json in addition to META.yml and MYMETA.yml.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Remove redundant .gitignore, .hgignore files

This is all taken care of by the root .gitignore file.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>